Known commercial mobile communication systems typically include a plurality of fixed base stations arranged in patterns whereby each base station transmits and receives over a plurality of frequencies. A mobile station within range of the base station can communicate with the external world (e.g., via the Public Switched Telephone Network (“PSTN)) through the base station using the frequencies. The area surrounding a base station in which mobile stations communicate with that base station is often referred to as a cell, with the base station generally positioned toward the center of the cell. Examples of known commercial mobile communications systems having cells include cellular communications systems, Personal Communications Systems (“PCS”), Global System for Mobile communication (“GSM”) systems, IS-136/Digital-American Mobile Phone systems (hereinafter “IS-136” or “D-AMPS”), and so on.
In an IS-136 system, a mobile station can communicate with the base station via a carrier frequency pair that includes two different (but paired) frequencies. The first frequency of the pair is the downlink (or forward) frequency where information is transmitted from the base station to the mobile station, and the second frequency of the pair is the uplink (or reverse) frequency where information is transmitted from the mobile station to the base station. Each carrier frequency pair is often referred to as a carrier or a channel, although the term channel is also used in different ways when a carrier can carry multiple channels (e.g., time-division multiple access (“TDMA”) channels, code-division multiple access (“CDMA”) channels, and so on). An IS-136 system can have 416 carriers, of which 395 carriers are available to carry voice traffic between a mobile station and a base station.
The carriers used by a base station are separated from one another in frequency to minimize interference. A cell's carriers are carefully selected so that adjoining cells do not transmit or receive on the same carrier frequencies. A mobile system operator can allocate to a base station a set of carriers with frequencies that are each separated from the next carrier by an integral number. For example, FIG. 1 shows a known frequency reuse pattern for base station cells, where each cell is assigned a set of carriers. Each cell can be allocated one of seven sets of carriers, where a cluster of seven cells as a whole is allocated all of the carriers. Thus, the frequency reuse pattern illustrated in FIG. 1 is typically referred to as having n=7 clusters. The cells are arranged and frequency sets can be allocated by assigning a first carrier set (e.g., carrier set 1) to a central cell 111 of a first cluster, and then assigning different carrier sets (e.g., carrier sets 2-7) to the cells of the first cluster surrounding that central cell. Thus, cell 111 can have carrier set 1, cell 112 can have carrier set 2, cell 113 can have carrier set 3, cell 114 can have carrier set 4, cell 115 can have carrier set 5, cell 116 can have carrier set 6, and cell 117 can have carrier set 7. Each of the carrier sets are also respectively allocated to the cells 141-147 of a fourth cluster adjacent to cells 111-117 of the first cluster. Portions of other adjacent clusters—such as cells 125 and 126 of a second cluster, cells 131 and 134-137 of a third cluster, and so—are also illustrated.
The assignment of carriers to carrier sets, and the assignment of carrier sets to cells, can be based on the number of different carrier sets (e.g., seven, four, and three carrier sets) and the number of available carriers. An IS-136 system having 395 voice carriers and using a frequency reuse pattern illustrated in FIG. 1 can have approximately 57 carriers per carrier set and cell. With seven different carrier sets, carrier set 1 can include carriers 1, 8, 15, 22, 29, 36 and so on; carrier set 2 can include carriers 2, 9, 16, 23, 30, 37 and so on; carrier set 3 can include carriers 3, 10, 17, 24, 31, 38 and so on; and so forth with respect to carrier sets 4-7.
FIGS. 2 and 3 illustrate other known frequency reuse patterns. In particular, FIG. 2 illustrates a frequency reuse plan having n=3 clusters. A first cluster can have three cells 211-213. Six clusters—such as a second cluster having cells 221-223, a third cluster having cells 231-233, a fourth cluster having cells 241-243, and so on—can be located adjacent the first cluster. Each cell of each cluster can be allocated a third of the available system carriers. FIG. 3 illustrates a frequency reuse plan having n=4 clusters. A first cluster can have four cells 411-114. Six clusters—such as a second cluster having cells 321-324, a third cluster having cells 331-334, a fourth cluster having cells 341-344, and so on—can be located adjacent the first cluster. Each cell of each cluster can be allocated a fourth of the available system carriers.
To allow a mobile station to transmit and receive communications as the mobile station moves from one cell to another, each cell is normally positioned with its area of coverage overlapping the areas of coverage of a number of adjacent and surrounding cells. As a mobile station moves from an area covered by a first base station to an area covered by another base station, mobile station communications (e.g., a voice call, a data link, etc) are transferred from the first base station to the other base station in an area where the coverage from the two cells overlaps. The transfer of a mobile station from communicating with one base station to communicating with a second base station is typically called hand off.
A cell can have at least two types of radio coverage. A first type of cell radio coverage is omnidirectional (i.e., azimuthally), where the cell has an antenna set that can communicate with mobile stations via each carrier of the carrier set allocated to the cell. A second type of cell radio coverage is sectored. FIG. 4 shows an illustration of a sectored cell. Cell 411 includes a plurality of sectors, including sectors 401, 402, and 403. Sectors are often referred to as an alpha sector, a beta sector, and a gamma sector. Cells are typically divided into three sectors, with each sector antenna set that covers a 120° sector. In a cell having three sectors, each sector antenna set can communicate with mobile stations via one-third of the carriers of the carrier set allocated to the cell so that each sector communicates over different carriers as compared to the other sectors of the cell.
Notwithstanding the use of frequency reuse patterns, interference between like carriers of different cells can occur. For example, referring again to FIG. 1, even though cell 131 is a knight's move away from cell 111 (i.e., cell 131 is up two cells and over one cell from cell 111), there can be interference between the carriers of cell 111 and cell 131. For example, within portions of cells 113, 137, and 136, there can be interference between a carrier 1 of cell 111 and a carrier 1 of cell 131. Such interference is typically called co-channel interference.
Co-channel interference can be caused by antenna patterns, power levels, carrier scattering, and wave diffraction that differ from cell to cell. Buildings, structures, mountains, foliage, and other physical objects can cause carrier signal strength to vary over the area covered by a cell. As a result, the boundaries (i.e., edges) at which the signal strength of a carrier falls below a level sufficient to support communications with a mobile station can vary widely from cell to cell. Thus, cells adjacent one another do not typically form anything like the precise geometric patterns illustrated in FIGS. 1-3. Cell coverages, however, must overlap to allow mobile stations to be handed-off between cells, and such overlapping, among other factors, can lead to co-channel interference.
In an IS-136 system, mobile stations are instructed to measure the signal strengths of various carriers and report the measured signal strengths to the mobile system. For example, referring again to FIG. 1, as a mobile station in communication with the base station of cell 111 moves through cell 111 toward cells 112 and 113, the mobile station can be instructed to measure the signal strengths of certain carriers of 111, 112, and 113 and report the measured carrier signal strengths to the mobile system via the base station of cell 111. When the signal strength reported by the mobile station with respect to the cell 111 carrier drops below a certain threshold (e.g., as the mobile station approaches the intersection of cells 111, 112, and 113), the mobile system will pick one carrier of the carriers measured and reported by the mobile station and instruct the mobile station to use that carrier for communications (e.g., instruct the mobile station to begin communicating with the base station of cell 113 or cell 112 via the appropriate carrier). In known IS-136 systems, mobile stations can monitor and report the carrier strengths of neighboring surrounding cells.
Mobile system operators generate frequency reuse plans to, among other things, reasonably minimize co-channel interference and reasonably maximize the likelihood that mobile stations will be successfully handed-off to a next cell as it moves away from its current cell. A first method of generating a mobile system frequency reuse plan is to use a frequency reuse pattern as illustrated in FIGS. 1-3. The efficiency of a frequency reuse plan can be increased by modifying the frequency reuse plan based on knowledge (subjective and/or objective) of the terrain covered by the frequency reuse plan. For example, FIG. 5 illustrates a reuse pattern that has been modified based on terrain characteristics. A first cluster includes cell 511-517. A mountain range 501 abuts the edge of the first cluster at the exterior edges of cells 513 and 514. The mountain range 501 attenuates the carrier signals transmitted by cells 512, 511, and 515. Thus, cells 522, 521, and 525 can use the same carrier sets used by cells 512, 511, and 515 with a reasonable minimization of co-channel interference. By reusing the carrier sets in a more compact manner, the frequency reuse plan illustrated in FIG. 5 is more efficient than the frequency reuse pattern illustrated in FIG. 1. A more efficient frequency reuse plan allows for greater system utilization (e.g., more mobile stations can be supported).
Frequency reuse plans can also be based on predictive methods using computer modeling. A computer model can predict carrier propagation areas based on antenna height, transmitter power, terrain characteristics, and so forth. Measured carrier data can also be used to create and modify frequency reuse plans. In an IS-136 system, mobile stations report received carrier strengths to the mobile switch coupled to the base stations. The reported carrier strength data can be used to determine how far carriers propagate.
Carrier propagation and co-channel interference can also be measured by receivers that measure received carrier strength as they are driven throughout areas of the mobile system during a so-called “drive test.” For example, during a drive test a specific test carrier is transmitted at each cell or sector of a cell involved in the interference testing. A scanning receiver is driven over the roads, highways and traveled byways of the system. The scanning receiver scans and measures the strength of the test carrier signal transmitted by each cell at the points of possible interference, and location determination equipment (e.g., a Global Positioning System (“GPS”) unit, a Loran unit, etc.) records the position of the scanning receiving. These strength measurements are then plotted and the expected interference points from different cells may be viewed graphically to determine whether sufficient interference exists to change the channel sets assigned to a particular area. This method of performing a drive test is often referred to as a “key-up” drive test because the test carrier is continuously “keyed-up” at each cell so as to be measured. A test carrier does not carry subscriber communications.
U.S. Pat. No. 5,926,762 (“the '762 patent”) describes another type of drive test in which a unique test carrier at each cell site is transmitted such that each cell site is transmitting a different test carrier. A scanning receiver is driven over the roads, highways and traveled byways of the system to measure the strength (typically the received signal power) of each test carrier transmitted by each of the cell sites while location determination equipment records the position of the scanning receiver. According to the '762 patent, transmitting a different test carrier at each cell eliminates interference that can complicate strength measurements when a single carrier is keyed-up at multiple cells for a drive test.
These known methods of performing drive tests to measure carrier strengths and predict co-channel interference require test carriers to be keyed-up to continuously transmit. Whether a single test carrier is keyed-up at a plurality of cells, or different test carriers are keyed-up at different cells, each method requires keying-up a test carrier. When a test carrier is keyed-up, it is not available to carry subscriber communications (e.g., voice traffic), and system capacity is diminished. Thus, key-up drive tests are typically conducted during the evening when demand for system capacity is lowest. In view of the forgoing, it can be appreciated that a substantial need exists for systems and methods that can advantageously provide for determining mobile communication system telephone carrier frequency propagation characteristics.